1. Field of the Invention
The present invention relates to a braking system for use in an automotive vehicles and particularly to a hydraulic braking system comprising a tandem master cylinder and a dynamic hydraulic braking pressure generator which outputs hydraulic power pressure supplied from a power source in response to the depression of a brake pedal.
2. Description of the Prior Art
In a conventional service braking system for an automotive vehicle, there is provided a plurality of hydraulic circuits connecting wheel brake cylinders mounted on road wheels with a hydraulic braking pressure generator such as a master cylinder, so that when one of the hydraulic circuits is broken or otherwise inoperative, normal braking operation is achieved by the rest of the hydraulic circuits. In general, a tandem master cylinder is used in a conventional dual circuit system.
In order to reduce the force required to operate a brake pedal during braking operation, the hydraulic braking system is provided with a servo unit which is referred as a servo or a booster and which utilizes compressed air, intake manifold vacuum (for a vacuum booster), or hydraulic pressure (for a hydraulic booster) as a power source. The hydraulic booster is a booster which actuates the hydraulic braking pressure generator such as the master cylinder by the hydraulic power pressure supplied from the power source in response to depression of the brake pedal. For example, the Japanese Patent Laid-open Publication No. 59-209948 has disclosed a system providing the hydraulic booster to the tandem master cylinder which operates as the ordinary tandem master cylinder while the hydraulic booster is not operated.
With use of such hydraulic booster, it has been proposed to use the hydraulic booster, in the hydraulic braking system, as a dynamic hydraulic pressure generator in addition to the master cylinder. In other words, a hydraulic pressure boosted by the booster (hereinafter referred to as boost pressure), in response to the depression of the brake pedal, is applied directly to a hydraulic circuit. For example, as shown in Japanese Patent Laid-open Publication No. 59-227552, boost pressure of the hydraulic booster is applied to rear wheel brake cylinders in a front-rear dual circuit system in order to reduce the stoke of the brake pedal.
Further, regarding the hydraulic braking pressure generator provided with the tandem master cylinder and the hydraulic booster, it has been proposed to connect one of the pressure chambers of the tandem master cylinder with the wheel cylinders of the one of the hydraulic circuits and to output the boost pressure of the hydraulic booster to the wheel cylinders of the other of the pressure chambers of the tandem master cylinder. Such hydraulic braking system is disclosed in Japanese Patent Laid-open Publication No. 62-155167 . According to this system, various effects such as the shortening of the stroke of the brake pedal and so on are obtained.
However, such prior art explained earlier (Japanese Patent Laid-open Publication No. 52-227552) is accompanied by a problem that if the hydraulic power pressure is lost during operation of the power source, the braking force of the rear wheels is lost, although the braking force of the front wheels can be maintained by the master cylinder.
On the contrary, according to the prior art explained later, in case of the loss of the hydraulic power pressure, the system functions as the ordinary tandem master cylinder and the braking force of all the wheels can be maintained. In the prior art explained later, however, since the tandem master cylinder is the type which utilizes a compensating port communicating with the reservoir and the sliding operation of a piston controls the open-closed compensating port, even though a second piston slides toward the first piston and the brake pedal at the first stage of the brake operation, the first piston, the first pressure chamber and the hydraulic circuit communicated with the first pressure chamber are not influenced. That is, the above movement of the second piston is not concerned with the shortening of the stroke of the brake pedal and the shortening of the stroke of the brake pedal is limited to directly effect the utilization of the hydraulic booster which is the dynamic hydraulic braking pressure generator. Accordingly, in this system, the first stage idle stroke exists when the first piston is operated and it is to further shorten the stroke of the brake pedal.
Further, in the prior art explained later, when the brake pedal is depressed suddenly, the two pistons are slidably moved and the hydraulic braking pressure is outputted from the two pressure chambers before the boost pressure of the hydraulic booster is supplied to the tandem master cylinder. Subsequently, the boost pressure of the hydraulic booster is supplied to the supply chamber of the second piston of the tandem master cylinder after two pistons are slidably moved. As a result, the urging force toward the brake pedal adds to the first piston which was sliding toward to the second piston and a reaction is generated in the brake pedal. This reaction does not cause a hindrance to the braking operation, but the brake feeling deteriorates.